A dynamic random access memory (DRAM) includes arrays of memory cells wherein each memory cell includes a memory cell capacitor and a memory cell access transistor. Each memory cell stores 1 bit of data, and the memory cell capacitor may have a capacitance of at least 30 fF to reduce soft errors due to .alpha. particle irradiation. As the size of memory cells have been reduced to increase integration densities, there have been efforts to provide predetermined capacitances with smaller capacitors.
The capacitance of a given capacitor can be determined using the equation listed below. ##EQU1## In this equation C is the capacitance of the capacitor, .epsilon..sub.o is the permeability of a vacuum, .epsilon..sub..tau. is the dielectric constant of the capacitor dielectric, A is the effective area of the capacitor electrode, and d is the thickness of the capacitor dielectric film. Accordingly, the capacitance can be increased by decreasing the thickness of the dielectric film, increasing the effective area of the capacitor electrodes, and increasing the dielectric constant of the capacitor dielectric.
When decreasing the thickness of a capacitor dielectric film to 100 .ANG. or less, the reliability of the capacitor may be reduced. In particular, Fowler-Nordheim currents may result when thin dielectric films are used in a large capacity memory device. The effective area of the capacitor electrode may be increased by using a three dimensional capacitor structure. The fabrication of a three dimensional capacitor structure may, however, be more complicated thus increasing production costs. In particular, three dimensional structures such as stack-type structures and trench-type structures have been applied to 4 MB DRAMs, but these structures may be difficult to apply to 16 MB or 64 MB DRAMs. Moreover, a stack-type capacitor may have a relatively steep step due to the height of the stack-type capacitor over the memory cell transistor. In addition, leakage currents may occur between trenches of trench-type capacitors when scaled down to the size required for a 64 MB DRAM.
Materials with relatively high dielectric constants include Yttria (Y.sub.2 O.sub.3), tantalum oxide (Ta.sub.2 O.sub.5), titanium oxide (TiO.sub.2). In addition, ferroelectric materials such as PZT (PbZr.sub.x Ti.sub.1-x O.sub.3) or BST (Ba.sub.x Sr.sub.1-x TiO.sub.3) have been used to provide relatively high dielectric constants. A ferroelectric material exhibits a spontaneous polarization phenomenon, and dielectric constants for ferroelectric materials can range from on the order of hundreds up to thousands. When using a ferroelectric material as a dielectric film for a capacitor, a thickness of hundreds of angstroms can provide a dielectric equivalent of a 10 .ANG. oxide film.
Methods for manufacturing dielectric films for DRAMs or nonvolatile memories have been proposed using a Perovskite-structured oxide, such as PZT (PbZrTiO.sub.3), BST (BaSrTiO.sub.3), or STO (SrTiO.sub.3). For example, a method for forming a BST film using chemical vapor deposition (CVD) is discussed in the reference by Takaaki Kawahara et al., entitled "Surface Morphologies and Electrical Properties of (Ba, Sr)TiO.sub.3 Films Prepared by Two-Step Deposition of Liquid Source Chemical Vapor Deposition", Japanese Journal of Applied Physics, Vol. 34, pp. 5077-5082, September 1995.
According to the Kawahara reference, the surface of a thin ferroelectric film may become nonuniform if the ferroelectric film is formed as a single film. The nonuniformity may result because of protrusions caused by a nonuniform density in nuclear generation. In addition, the Kawahara reference states that the nonuniformity can be reduced by depositing a buffer layer of 60 .ANG. while maintaining the substrate at a temperature in the range of 420.degree. C., performing a first anneal in a nitrogen N.sub.2 ambient, depositing a main layer, and performing a second anneal in an nitrogen N.sub.2 ambient to crystallize the thin film. The Kawahara process, however, may be complicated because two annealing steps are used to obtain a crystal form of BST.
In addition, Japanese publication No. Hei 6-21337 discusses a dielectric device capable of reducing leakage current of a ferroelectric film due to a fatigue phenomenon. According to the reference, when an ABO.sub.3 structured ferroelectric material such as PZT is used as a dielectric film of a capacitor, the ratio of element composition of a cubic "A" with respect to the element composition of cubic "B" ([A]/([A]+[B])) in the boundary surface between the ferroelectric material and an electrode is different from that in a bulk region of the ferroelectric material. To form such a dielectric device, a source component of the ferroelectric film or the condition of a reactive chamber within which deposition of the ferroelectric film is performed is changed, and the manufacturing process may become complicated. Notwithstanding the structures discussed above, there continues to exist a need in the art for improved methods of forming dielectric films having relatively high dielectric constants.